This invention relates to an optical system which generates two or more identical images of an object line on the object side of a lens. The lens focuses the images or portions thereof, onto desired portions of an image plane. More preferably, the imaging plane comprises two or more linear photosensor arrays. The use of arrays of solid-state photosensors in applications such as video camera and facsimile transmission is known in the art. An object plane is scanned and the image of the scanned line is focused onto a photosensor array. The array comprises a plurality of photosensitive elements, such as photodiodes or metal-oxide-semiconductor (MOS) photodetectors arranged in a line. Each element produces an electrical signal representative of the irradiance of the portion of the image incident thereon so that the electrical signals, in combination, constitute an electrical representation of the scanned line. These signals can then be transmitted to remote locations and utilized as desired.
The semiconductor fabrication techniques currently employed have two characteristics which limit certain types of scanning applications. The maximum linear dimension of an array currently achievable with high production yields is approximately one inch; this array can be packed with an in-line array of 2000 photosensors. A single array, if produced at these outer limits is quite costly and is still not sufficient to provide sufficient resolution of scan lines which exceed more than 81/2 inches in length. The optimum solution to this problem would be to butt together two or more arrays so that all the information along the line being scanned is sensed. However, this is not presently possible because even with the most efficient manufacturing techniques, there is a small space at the abutting interface resulting in some separation between the last element on one array and the first element on the next array. This space results in some unavoidable loss of information.
One solution to this problem is to optically abut the diodes. A typical system is disclosed in IBM Technical Disclosure Bulletin, Vol. 20, No. b 1, June 1977. As disclosed therein, a beam-splitter on the image side of a focusing lens produces a first and a second image. The arrays are located so that each array senses only a portion of the scanned line but they optically function as a single array. Price in U.S. Pat. No. 4,005,205 discloses a prism for splitting an optical path into twin paths on the image-side of a lens. These and other known methods of image-side beam-splitting however, are radiometrically inefficient. Between 50 and 75 percent radiometric losses can be experienced requiring compensation in the form of slower scan speed or greater scan illumination requirements. Further, when the full aperture of the lens is not used for each image, resolution degradation generally occurs.
U.S. Pat. No. 4,044,384 discloses several optical configurations wherein the scan line is divided into two or more portions and the images of each portion are focused onto respective photosensitive arrays or onto the same array sequentially. The signals are then combined electrically to reproduce the entire scan line. These configurations have the undesirable features of requiring additional lenses and, in the case of the single array, a shutter arrangement.
The present invention is directed to an optical system located on the object side of a lens which generates two or more linear images from the common object line being scanned. Since each image is, in effect, an original image, the significant radiometric losses of the image side beam-splitters discussed above is avoided. Since the full lens aperture is used for each image, resolution losses are also avoided.
It is known in the steroscopic art to form two separate stereoscopic negatives by using beam-splitters on the object side of the lens. See, for example, U.S. Pat. Nos. 2,403,733; 862,354; and 2,736,250. The particular requirements of these systems serve to distinguish them over the present optical system. For example, in order that the two separate images be perceived by the eye as a three-dimensional scene, the images that are produced must be slightly dissimilar. The present system requires identical images. The stereo systems disclosed require elaborate mirror/prism configurations to reduce the keystoning effect; the present system uses a simple two mirror system forming two principal ray paths with a small angular separation betweem them, as viewed from the line source being scanned. The present system uses a controlled zone of illumination at the object to prevent overlapping images. The stereo system is generally designed for broadly illuminated objects at large distances from the main lens and must therefore incorporate special field limiting stops on the mirror or prisms. Functionally, the present system in its preferred embodiment differs in that it is directed towards forming separate images of a scanned line and focusing the image onto photosensor arrays.